Then he moved to more southern Crowborough, Sussex and specialized on photographing star clusters and nebulae. In 1888, he obtained a photograph of the Andromeda Nebula M31, well showing its spiral structure. Roberts believed that M31 and other spiral "nebulae" were solar systems in formation, with the satellite galaxies M32 and M110 being planets in formation.
Isaac Roberts published his photographes in three volumes of a series, Photographs of Stars, Star Clusters and Nebulae. The first volume was published in 1893, the second in 1899, and the third one posthumously by his widow in 1928.
According to Kenneth Glyn Jones, his 20-inch reflector is now in the Science Museum in South Kensington, London (England).